A wireless mobile communication system divides a total resource such as frequency bands or data rates into many smaller ones, schedules them according to respective users' requests, and thus shares the resource by using multiple access methods such as time division, frequency division, and code division.
The method for a plurality of mobile stations to transmit intermittent data through an uplink common channel for the purpose of effective usage of resources is a random access method for allowing all the mobile stations to have transmission rights and transmit data randomly.
One of the random access methods is to allow a mobile station to transmit a preamble to a base station in advance. The preamble is generated by using a randomly selected code from among distinguishable codes. After having received a corresponding access grant on the preamble from the base station, the mobile station transmit the actual random access data.
The above-noted method for transmitting random access data allows the preamble and the data to be transmitted through a common random access channel, and additionally requires resource assignment.
Further, one of methods for responding to the preamble transmitted from the mobile station is to use the code which is received when an access grant signal is generated, and re-transmit the preamble. Another one is to use code and channel information which is captured at the time of receiving the preamble.
Since a lesser amount of resources is needed for transmitting the preamble, it is allowable to assign a lesser amount of resources irrespective of a random access request by the mobile station. However, it is inefficient to assign the transmission resource for transmission of random access data irrespective of a random access request by the mobile station.